IRAC is located slightly off the optical axis of the Spitzer telescope. As a result there is a slight amount of distortion in the field of view. The pixels do not all subtend the same projected solid angle on the sky. The variation in pixel solid angle size is roughly 1.5%.

This is important to understand because the BCD images are calibrated in units of surface brightness (specifically MJy/sr). In order to properly measure fluxes, one must take into account the size of the pixels. Failure to do so could induce photometric errors at the 1% level, depending on location on the array. Unfortunately, only the newest photometry software can read the new FITS-standard WCS distortion keywords written in the BCD headers and properly account for the sizes of the pixels. Instead, virtually all software assumes that the pixels are in units of flux per pixel, which is equivalent to units of flux density/sr if the pixels are the same size.

The simplest solution to this problem is to reproject the images onto an equal (or nearly so) projection system using suitable software that can understand the distortion keywords in the WCS header (e.g., MOPEX). MOPEX also has the significant advantage that it understands how to properly handle surface brightness images during coaddition. After processing, the pixels will all subtend the same solid angle, and hence any standard photometry software can produce the correct result.

However, some observers might prefer alternative approaches. Therefore, we supply maps of the pixel size. The FITS images below contain the pixel solid angle of the IRAC pixels relative to the center pixel. The nominal pixel width is 1.221, 1.213, 1.222, and 1.220 arcseconds.